REST API: Slash existing meta values when comparing with incoming meta upates.

When comparing the old and new values for a meta key being set, ensure both values are sanitized using the same logic so that equal values match.

props boonebgorges, dcavins, MattGeri, pilou69, TimothyBlynJacobs, kadamwhite.

Merges [43740] to trunk.

Fixes #42069.
Built from https://develop.svn.wordpress.org/trunk@44113


git-svn-id: https://core.svn.wordpress.org/trunk@43943 1a063a9b-81f0-0310-95a4-ce76da25c4cd